Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

Compound A is an organic substance containing carbon, hydrogen and oxygen. When $0.240\,\text{g}$ of its vapour is passed slowly over a large quantity of heated copper(II) oxide, $\text{CuO}$, compound A is completely oxidised to carbon dioxide and water. Copper is the only other product formed. The products are collected and it is found that $0.352\,\text{g}$ of $\text{CO}_2$ and $0.144\,\text{g}$ of $\text{H}_2\text{O}$ are obtained.
(a)

Give your answers in this section to three decimal places.

(a(i))

Work out the mass of carbon contained in $0.352\,\text{g}$ of $\text{CO}_2$. Use that result to find the amount, in moles, of carbon atoms in $0.240\,\text{g}$ of A.

(a(ii))

Work out the mass of hydrogen present in $0.144\,\text{g}$ of $\text{H}_2\text{O}$. Use that value to calculate the amount, in moles, of hydrogen atoms in $0.240\,\text{g}$ of A.

(a(iii))[6]

Use your answers to work out the mass of oxygen in $0.240\,\text{g}$ of A. Use this value to calculate the amount, in moles, of oxygen atoms in $0.240\,\text{g}$ of A.

(b)[1]

Use your answers to (a) to determine the empirical formula of A.

(c)

A $0.148\,\text{g}$ sample of A was vapourised at $60^{\circ}\text{C}$, and its vapour occupied $67.7\,\text{cm}^3$ at a pressure of $101\,\text{kPa}$.

(c(i))

Use the general gas equation $pV = nRT$ to calculate $M_r$ for A.

(c(ii))[3]

From this, calculate the molecular formula of A.

(d)[2]

Compound A is a liquid that shows no reaction with 2,4-dinitrophenylhydrazine reagent or with aqueous bromine. Suggest two structural formulae for A.

(e)[1]

Compound A contains only carbon, hydrogen and oxygen. Explain how the details on the opposite page about A reacting with $\text{CuO}$ support this statement.

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